Java基础 内部实现
public static int parseInt(String s, int radix)
throws NumberFormatException
{
if (s == null) {
throw new NumberFormatException("null");
}
//判断进制解析范围，只支持2-36进制，默认是十进制，redix = 10
if (radix < Character.MIN_RADIX) {
throw new NumberFormatException("radix " + radix +
" less than Character.MIN_RADIX");
}
if (radix > Character.MAX_RADIX) {
throw new NumberFormatException("radix " + radix +
" greater than Character.MAX_RADIX");
}
int result = 0;//解析结果
boolean negative = false;//负号标记
int i = 0, len = s.length();
int limit = -Integer.MAX_VALUE;//解析结果的阈值，即不能解析出整型能表示的最大数
int multmin;
int digit;
if (len > 0) {
//以下为符号检测，检测第一个字符是否是负号，并根据负号更新解析限定值，即解析结果不能超过int型最大值
char firstChar = s.charAt(0);
if (firstChar < '0') { // Possible leading "+" or "-"
if (firstChar == '-') {
negative = true;
limit = Integer.MIN_VALUE;//如果是负数，限定值为最小的整数
} else if (firstChar != '+')
throw NumberFormatException.forInputString(s);
if (len == 1) // Cannot have lone "+" or "-"
throw NumberFormatException.forInputString(s);
i++;
}
multmin = limit / radix;
while (i < len) {//循环解析每个字符
// Accumulating negatively avoids surprises near MAX_VALUE
digit = Character.digit(s.charAt(i++),radix);//字符转换为数字
if (digit < 0) {//解析出了单个负数，抛出异常
throw NumberFormatException.forInputString(s);
}
if (result < multmin) {//结果解析已经超出限制了，因为后面还要继续*10，这里的result如果已经比Integer.MAX_VALUE/10还要大，那么数字就超了，抛出异常
throw NumberFormatException.forInputString(s);
}
result *= radix;//乘以10
if (result < limit + digit) {//与上一步一样，防止超出Integer表达范围
throw NumberFormatException.forInputString(s);
}
result -= digit;
}
} else {
throw NumberFormatException.forInputString(s);
}
return negative ? result : -result;//取相反数返回
}